Catheters are tube-like members inserted into the body for diagnostic or therapeutic reasons. One of the therapeutic procedures applicable to the present invention is known as percutaneous transluminal coronary angioplasty (PTCA). This procedure can be used, for example, to reduce arterial build-up of cholesterol fats or atherosclerotic plaque. Catheters must have sufficient stiffness to be pushed through vessels as well as sufficient rigidity to provide a high degree of torsional control. Stiffness or rigidity in the catheter tip poses the danger of puncturing or otherwise damaging a vessel as it twists through the vascular system. It is therefore desirable for catheters to have a soft or flexible distal tip. Examples of such soft tip catheters are known in the art.
The trend toward catheters with larger inside diameters and softer distal tip segments results, however, in a substantially weaker bond between the soft tip and the distal catheter shaft because of the thinner wall thickness and lower tensile strength of the softer materials. The following methods of tip attachment are known in the art.
U.S. Pat. No. 4,531,943 to Van Tassel et al for a "Catheter with Soft Deformable Tip" discloses an embodiment at col. 4, lns. 43-47 wherein the tip member comprises a tubular sleeve which surrounds and fits by means of a lap joint at the distal end of the catheter body and the sleeve extends beyond the distal end thereof by a predetermined length.
U.S Pat. No. 4,551,292 to Fletcher et al for a "Method for Making a Catheter with a Soft, Deformable Tip" discloses a method of tip construction whereby the tip is heated to soften the plastic, then a forming tool is inserted into the catheter lumen distal end and advanced to the point where an annular protuberance of a predetermined outside diameter is forced into the lumen so as to stretch and shape the plastic to conform to the forming tool. The distal end is then cooled and the forming tool removed, leaving a soft, collapsible segment integrally formed at the distal end of the catheter.
U.S Pat. No. 4,563,181 to Wijayarathna et al for "Fused Flexible Tip Catheter" discloses a soft tip formed from a blend of the nylon of the body portion with an ester linked polyether-polyamide co-polymer which is soft and rubbery to render the tip soft to avoid injury to a blood vessel. The tip is fused or welded at a butt joint to the tubular body which is made of a stiffer nylon.
U.S Pat. No. 4,636,346 to Gold et al for a "Preparing Guiding Catheter" discloses an embodiment in col. 5, lines 12-20 wherein the tip portion may be an initially separate member that is affixed to the elongated tubular body by suitable means, such as by heating, by other energy sources, and/or by adhesives or the like. Such assembly can be assisted by the use of a length of shrinkable tubing that is placed over the joint location prior to and during the assembly operation in order to enhance the smoothness and strength of the joint.
U.S. Pat. No. 4,863,442 to DeMello et al for a "Soft Tip Catheter" discloses an embodiment at col. 5, lines 30-52 wherein a sleeve of shrink film is placed over the polyurethane tube and the distal end of the jacket and overlaps the shoulder. With the sleeve of shrink film in place, the distal end of the assembly is heated to a temperature and for a time sufficient to cause the soft polyurethane tube to flow and fill the gap along with any other gaps which may exist between it and the shoulder, outer surface of the core, and the outer surface of the mandrel. The time and temperature is a function of the particular polyurethane used. With one material tested, the temperature was approximately 320 degrees for a duration of approximately five minutes. As the film shrinks under the application of heat, it will somewhat compress the polyurethane and cause it to conform closely to the contours of the mandrel. After the assembly cools, the mandrel may be removed and the shrink film should be stripped from the assembly.
U.S Pat. No. 4,886,506 to Lovgren et al for a "Soft Tip Catheter" discloses a method of achieving a lap joint at col. 2, lns. 60-62 defining a frusto-conical profile on the distal catheter shaft to which the tip is subsequently fused by RF welding, resulting in a lap joint. The tip has a size and shape adapted to be placed over the tapered portion coaxially, and a composition softer than that of the distal end portion that is suitable for bonding to the fustoconically-shaped outer surface.
U.S. Pat. No. 4,899,787, issued to Ouchi et al., discloses a flexible tube having two or more tube sections which are bonded to a tubular core which comprises one or more fabric mesh tubes and one or more metallic tubular spirals. The tube sections are butted together and then fused to the tubular core. A catheter utilizing this tubular core structure possesses undesirable stiffness and rigidity because of the presence of mesh tubes and metallic tubular spirals at the distal end of the catheter shaft. As a result, the tubular core poses the danger of puncturing or otherwise damaging a vessel as the catheter is manipulated through the vascular system.
U.S. Pat. No. 5,078,702 to Pomeranz for a "Soft Tip Catheter" discloses a tip welded to a tubular body. The tip has an inner sheath of a rigid polymeric material encapsulated by an outer sheath of a flexible polymeric material. The inner sheath of both the body and tip portions are formed from the same polymeric material.
U.S. Pat. No. 5,160,559 to Scovil et al for a "Method for Forming a Guide Catheter Tip Bond" discloses setting a mating distal end of a tubular member against a mating proximal end of a soft, deformable tip to form a butt joint. The butt joint is then softened to render the mating proximal and distal ends of the deformable tip and tube flowable. The tubular member and the deformable tip are then oscillated and advanced into one another along a longitudinal axis such that the materials of the mating proximal and distal ends flow into one another creating a connection zone which solidifies to form a lap joint tip bond.
U.S. Pat. No. 5,234,416, issued to Macaulay et al., discloses a distal soft tip comprising at least two relatively short, coaxially disposed flexible tubular elements. The "first tubular element" 17 is secured to the "distal section" 13 of the catheter shaft, and the "second tubular element" 18 which is softer than the "first tubular element" 17 is secured to the "first tubular element" 17. The "first tubular element" 17 incorporates a radiopaque filler to make the distal tip fluoroscopically observable. See col. 5, In 32-35. The "first tubular element" 17 has a durometer in the range of Shore 80A to 100A while the "second tubular element" 18 has a durometer in the Shore 70A to 90A range. See col. 6, In 54-59. The distal end of the catheter shaft has a circumferential shoulder over which the proximal end of the first tubular element, which is stepped to mate with the shoulder, is placed. The proximal end of the .sup."second tubular element" 18 is abutted against the distal end of the "first tubular element" 17. The short tubular elements are joined with the distal end of the catheter shaft by means such as melt fusing or adhesive bonding.
The joints employed to bond the tubular elements of the '416 Macaulay patent suffer from the same problems as the above-referenced prior art. The overlapping joint of the "first tubular element" 17 with the "distal section" 13 of the catheter shaft, similar to the '943 Van Tassel patent, supra, is undesirable because it creates a stress concentration area at the distal end of the catheter shaft in a plane perpendicular to the longitudinal axis of the catheter shaft. The effect of this stress concentration is an unacceptably low bond strength between the catheter shaft and the "first tubular element" 17 when the wall thickness of the catheter shaft is less than 0.3 min. Further, the butt joint design of the "second tubular element" 18 with the "first tubular element" 17, similar to the '181 Wijayarathna patent, supra, does not yield adequate bond strength because of the low surface area of contact and the stress concentration area at the junction of the "first and second tubular elements" 17 and 18 in a plane perpendicular to the longitudinal axis of the catheter shaft. The effect of the low surface area and the stress concentration is inadequate bond strength when the catheter shaft wall thickness is less than 0.3 mm and when soft, typically low tensile strength materials, such as Shore 70A to 90A TECOFLEX.RTM. are used for the "second tubular element" 18. The tensile strength ratio of the "first tubular element" 17 TECOFLEX.RTM. of EG93A-HT60 compared to the "second tubular element" 18 TECOFLEX.RTM. of EG80A is 126% as derived from the manufacturer's specification.